Re: [Linphone-developers] Using oRTP to send supplemental data besides the audio

[Steve Strobel]

At 04:40 PM 12/18/2008, Vadim Lebedev <address@hidden> wrote:

> FMTP is not part of RTP actually.
> It is simply an attribute in SDP  packet describing payload-specific
> parameters and options which are acceptabel for a give endpoint.

I don't know anything about SDP, but would guess from what you wrote 
that it is used specifically for establishing a connection and 
wouldn't be appropriate for passing other data.

> BTW how do you establish a session between two endpoints?
> If you use SIP, the best approach to send application specific data is
> to use SIP MESSAGE, INFO or OPTIONS requests

I am not (yet, at least) using SIP.  I took the oRTP send/receive 
examples and modified them to work with audio fifos that I already 
had.  The VoIP features are only a piece of a larger system used for 
connecting disparate radio systems together.  The code to establish a 
session just sets the local IP port number and remote IP 
address/port, plus a few RTP stack settings (scheduling options, 
etc).  I quit streaming RTP data each time a radio unkeys, and detect 
that loss of data at the other end to signal the corresponding radio 
there to quit transmitting.

Using SIP at least when I need to pass the extra data along with the 
audio might be a good option.  There are a few things that might 
cause new complications:

Handling PTT.  I need to have a way to signal when a radio keys and 
unkeys.  If SIP already has logic for handling that (such as might be 
needed for PTT cell phones), this might be easy.  Otherwise, I could 
place and hang up a call each time a radio keys and unkeys.  I need 
to be able to pass data even when no one is talking;  I suppose SIP 
already does that for sending presence information.

Is there a reasonably easy way to use the SIP stack from Linphone as 
a library?  Since the rest of my system is based on using fifos to 
pass audio streams, I don't think I can just run Linphone 
directly;  it would need to use ALSA or something to handle the 
audio, and once started, there wouldn't be an easy way to get the 
extra data I need to pass to and from the Linphone executable.  I 
suppose I could hack Linphone itself to change the audio I/O and 
adding the data passing stuff.

Thanks for your help thinking through the options,

Steve



---
Steve Strobel
Link Communications, Inc.
1035 Cerise Rd
Billings, MT 59101-7378
(406) 245-5002 ext 102
(406) 245-4889 (fax)
WWW: http://www.link-comm.com
MailTo:address@hidden